The separation of various components found in liquids or gases may be effected in a multitude of processes, the techniques for effecting the separation utilizing asymmetric or composite membranes including selective permeation, ultrafiltration or reverse osmosis. A particular example of the latter type of separation involves a desalination process in which water which is rendered potable or suitable for other purposes is obtained from sea water, contaminated water, brackish water or brine. This process is of especial value in areas of the world where the water found in the area is brackish or is saline in nature. The desalination of this water is necessary in order to provide large amounts of potable or relatively nonsalty water for industrial, agricultural or home use. The desalination of the water is effected by forcing the water through a reverse osmosis membrane whereby the purified water is passed through the membrane and recovered, while the contaminants or salts do not pass through the membrane, thus, in effect, being rejected by the membrane and recovered as the retentate.
A reverse osmosis membrane, in order to be utilized for such a purpose, must possess certain characteristics applicable to the process. For example, the membrane must have a very high salt rejection coefficient. In addition, another important characteristic and a problem which must be addressed when utilizing the membrane, is the ability of the membrane to be resistant to chlorine attack. Another important factor which is present in the use of a reverse osmosis membrane is that said membrane also possess a high flux characteristic, that is, the ability to pass a relatively large amount of water through the membrane at relatively low pressures. If a membrane possesses these desirable characteristics, it will be commercially feasible in its applicability to the desalination process.
Reverse osmosis membranes have been prepared and used from a wide variety of known polymeric materials. While many of these polymeric materials possess the ability of reducing the concentration of a solute to where the salt rejection capability is in excess of 98%, some do not possess the necessary flux rate whereby the volume of water which is required to be produced by the membrane per unit of membrane surface is sufficient for the application of the technology.
As was hereinbefore set forth, many prior U.S. patents describe various membranes which are useful in desalination processes. For example, U.S. Pat. Nos. 3,567,632, 3,600,350, 3,710,945, 3,878,109, 3,904,519, 3,920,612, 3,951,815, 3,993,625 and 4,048,144 illustrate various semipermeable membranes prepared from polyamides. Likewise, U.S. Pat. Nos. 3,260,691 and 3,480,588 disclose coating compositions which are obtained from the condensation products of aromatic primary diamines and aromatic tricarboxylic acid derivatives.
Inasmuch as the semipermeable membrane which is used for the desalination process should be relatively thin in nature in order to provide a desirable flux rate, it is necessary, in many instances, that the reverse osmosis membrane be composited or laminated on a porous backing support material. This porous support backing material should in itself possess certain characteristics which make it desirable for such a use. For example, the porous support material should possess pore sizes which are sufficiently large enough so that the water or permeate can pass through the support without affecting or lessening the flux rate of the entire composite. Conversely speaking, the pore size should not be large enough so that the thin composite semipermeable membrane will tend to fill up or penetrate too far into the pores, thus distorting the shape of the thin film membrane with the attendant possibility of rupturing the membrane when operated under high pressure, thus causing said membrane to lose its effectiveness in the reverse osmosis process.
In addition to the aforementioned U.S. patents, another U.S. patent, namely U.S. Pat. No. 4,277,344, discloses an interfacial synthesized reverse osmosis membrane. This membrane is prepared from an interfacially polymerized aromatic polyamine which has been prepared from an essentially monomeric polyacyl halide and an essentially monomeric arylene polyamine. The composite membrane is prepared by coating a support material with a liquid layer comprising an aqueous solution containing the polyamine reactant, contacting the liquid layer with essentially monomeric volatilizable polyfunctional acyl halide dissolved in a liquid aliphatic or liquid halogenated aliphatic solvent and drying the product formed thereby to form the desired membrane. In addition, the membrane may then be treated with an oxidizing agent and chlorine or a chlorine releasing agent to improve its chlorine resistance. The patent teaches that the membrane contains a plurality of sites having the formula: EQU Ar(CONH--).sub.2 COOH
in which Ar represents the aromatic nucleus residue of the polyfunctional aryl halide. In addition, the membrane is described as being lightly cross-linked in nature. The reaction is effected in the absence of any surface active agents and acid acceptors, the patentee stating that these compounds do not appear to provide any advantages in the context of the invention and that it is preferred to carry out the interfacial polymerization without the presence of surface active agents or acid acceptors.
Furthermore, the structure of the membrane will be dependent upon the water provided for in the aqueous solution to serve as a reactant and states that the aryl halide groups on the polyfunctional aryl halide are in a competitive state during the reaction with the aqueous solution of the polyamine. The patentee theorizes that the acyl halide groups can react either with water or with the primary amine groups or conversely that a sequential reaction occurs in which hydrolysis precedes condensation with an amine group.